Experimental analysis of subsurface integrity during fine turning of OFE copper for radiofrequency cavity manufacturing

A Camelin,G Poulachon,A D’Acunto,S Atieh,P Naisson

doi:10.1016/j.jmatprotec.2021.117483

Abstract

Machining oxygen-free electronic (OFE) copper could be challenging but is not widely studied because few industrial or critical components requires to master the machined sub-surface characteristics. CERN radio frequency cavities are one of the applications, especially because the turned surface is not the functional one of the final products. The niobium coating post process, which gives superconductive properties to the cavity, largely depends on the machined surface characteristics. The present study relies on an experimental approach of the cutting process, through thermal and mechanical probing of high precision, pollution free, turning. Cutting forces and thermal load on the tool are detailed for finish turning. The critical uncut chip thickness, defined at macroscale as the limit between cutting and ploughing behavior, is also a frontier at microscale. Consequently, surface integrity is evaluated by advance microstructural analysis (EBSD and FIB), imposed by the thinness of the affected layer. Grain recrystallization appears in the first 0.6 micrometers below the surface and deformed grains are observed up to 4 micrometers for cutting regime, while the thickness of the layers is three time larger in case of ploughing regime. Hence surface integrity of OFE copper finish turning is characterized and optimal cutting conditions are defined. The research shows that simple cutting tests can quickly narrow down to optimal cutting condition, which are then confirmed through metallurgical analysis, even in the edge case of pure OFE copper, hence relevant to other material.

Highlights

In addition to the Large Hadron Collider impact on fundamental nuclear physics, CERN houses numerous so-called fixed-target experi ments
The HIE-ISOLDE experiment aims to increase the energy and efficiency of the ISOLDE nuclear facility by upgrading its accelerating components, through thirty-two 40 MV superconducting quarter-wave resonators (Fraser et al, 2017). For these newly designed cavities, the niobium-on-copper technol ogy was selected. It implies the use of magnetron sputtering of a thin (2–10 micrometers) layer of pure RRR300 Niobium on an high purity copper substrate obtained after machining process as shown on Fig. 1 (Sublet et al, 2014)
The selected method to compare macroscopic parameter is based on the tool material pair (TMP) method defined by AFNOR standard (AFNOR, 1997)

Summary

Introduction

In addition to the Large Hadron Collider impact on fundamental nuclear physics, CERN houses numerous so-called fixed-target experi ments. The HIE-ISOLDE experiment aims to increase the energy and efficiency of the ISOLDE nuclear facility by upgrading its accelerating components, through thirty-two 40 MV superconducting quarter-wave resonators (Fraser et al, 2017) For these newly designed cavities, the niobium-on-copper technol ogy was selected. Surface and subsurface characteristics are linked to the machining process parameters, such as tool type, cutting data, or lubrication modes (Bellows and Tishler, 1970). In the HIE-ISOLDE cavity manufacturing scheme, a chemical etching treatment is carried out to remove between 15 μm–40 μm of the surface before the niobium coating (Sublet et al, 2014) This is made to prevent any contamination of the layer from the bulk material. The machining process to obtain OFE copper cavity in case of super finish turning will be qualified

Copper machinability issue

Experimental setup

Mechanical load induced by turning of OFE copper

Conclusions

Findings

Camelin